1. Field of the Invention
The present invention relates to an optical unit using an acousto-optic deflector (AOD), a method for controlling the AOD, and a holographic apparatus that record and reconstruct data on a holographic recording medium using interference fringes generated by a signal light beam and a reference light beam.
2. Description of the Related Art
Recently, a holographic recording and reconstructing method for recording data using interference fringes generated by a signal light beam and a reference light beam has been developed.
FIG. 8 illustrates a typical internal configuration of an existing holographic recording and reconstructing apparatus 50. In particular, FIG. 8 illustrates the structure of an optical system of the holographic recording and reconstructing apparatus 50.
As shown in FIG. 8, a holographic recording medium HM has, for example, a disc shape. The holographic recording medium HM is placed at a predetermined location inside the holographic recording and reconstructing apparatus 50 and is rotatingly driven by a spindle motor 30. The holographic recording and reconstructing apparatus 50 records and reconstructs data on the rotating holographic recording medium HM.
In existing recordable discs, such as compact discs-recordable (CD-Rs) and digital versatile discs-recordable (DVD-Rs), a groove is formed so that a data recording and reconstructing apparatus can control the position of a laser spot on the disc even when no data are recorded. Similarly, holographic recording media HM have a layer including a groove in order to control the position of a laser spot.
A method for recording and reconstructing data on a holographic recording medium HM is schematically described next.
In order to record data, a signal light beam subjected to spatial light modulation in accordance with data to be recorded and a reference light beam different from the signal light beam are emitted onto a holographic recording medium. Thereafter, interference fringes (a diffraction grating) generated by the two beams are recorded in the holographic recording medium HM. Thus, the data is recorded.
In contrast, in order to reconstruct the data, a reference light beam is emitted onto the holographic recording medium HM. By emitting the reference light beam in this manner, a diffraction light beam in accordance with the interference fringes formed in the holographic recording medium HM can be obtained. That is, by emitting the reference light beam, a reconstructed image (a reconstruction signal light beam) in accordance with the recorded data can be obtained.
Subsequently, by detecting the reconstructed image using an image sensor, such as a charge coupled device (CCD) sensor or a complementary metal-oxide semiconductor (CMOS) sensor, the recorded data can be reconstructed.
The holographic recording and reconstructing apparatus 50 shown in FIG. 8 has a configuration that realizes the above-described recording and reconstructing method.
The holographic recording and reconstructing apparatus 50 includes a main laser 2. The main laser 2 serves as a light source of a laser beam used for recording and reconstructing data on a holographic recording medium HM. For example, the main laser 2 is designed so as to output a laser beam having a wavelength of about 405 nm (i.e., a blue-violet laser beam).
The recording and reconstructing laser beam emitted from the main laser 2 passes through an isolator 3. The recording and reconstructing laser beam then passes through an acousto-optic modulator (AOM) 51 and an AOD 4 disposed in an image stabilization function unit 53 (the image stabilization function unit 53 is described in more detail below). Thereafter, the diameter of the laser beam is controlled to a desired value by a beam expander 7. The laser beam is then reflected off a mirror 8 and a mirror 9. The laser beam is then made incident on a spatial light modulator (SLM) 10.
The SLM 10 performs spatial light modulation on the incident recording and reconstructing laser beam so as to generate the above-described reference light beam and a signal light beam. For example, a diffraction spatial light modulator including a plurality of micromirrors arranged therein or a spatial light modulator using a liquid crystal panel can be used for the SLM 10. These spatial light modulators can perform spatial light modulation on a pixel-by-pixel basis. In this way, the above-described signal light beam based on data to be recorded and reference light beam having a predetermined intensity pattern can be generated.
Each of the pixels of the SLM 10 is controlled by a recording modulation unit 28.
As described above, when data is recorded, a signal light beam having a pattern in accordance with the data to be recorded and a reference light beam are generated. When data is recorded, these signal light beam and reference light beam generated by the SLM 10 are emitted onto the holographic recording medium HM via a beam splitter 11, a relay lens 12, a relay lens 13, a dichroic mirror 14, a mirror 15, and an objective lens 16. In this way, the signal light beam and the reference light beam interfere with each other, so that a diffraction grating (a hologram) in accordance with the above-described signal light beam pattern is generated in the holographic recording medium HM. That is, the data can be recorded.
The objective lens 16 is supported so as to be movable by a focus actuator 17A in focusing directions (directions towards and away from the holographic recording medium HM). The objective lens 16, the focusing actuator 17A, and the mirror 15 are disposed so as to be integrally movable by a tracking actuator 17B in tracking directions (the radial directions of the holographic recording medium HM).
In addition, when data is reconstructed, a reference light beam is generated by the SLM 10. The generated reference light beam travels along the above-described light path and is emitted onto the holographic recording medium HM. By emitting the reference light beam in this manner, a diffraction light beam (a reconstructed image) in accordance with the interference fringes can be obtained from the holographic recording medium HM, as described above. The diffraction light beam serves as a returning light beam from the holographic recording medium HM. The returning light beam is made incident on the beam splitter 11 via the objective lens 16, the mirror 15, the dichroic mirror 14, the relay lens 13, and the relay lens 12. The returning light beam from the holographic recording medium HM is reflected by the beam splitter 11 and travels through a relay lens 18 and a relay lens 19. The returning light beam is then led to an image sensor 20, such as a CCD sensor or a CMOS sensor.
The image sensor 20 detects the light intensity pattern of the returning light beam (the reconstructed image). That is, by detecting the light intensity pattern, a readout signal for data recorded in the holographic recording medium HM can be obtained.
A data reconstruction unit 29 receives such a readout signal from the image sensor 20 and performs a predetermined decoding process. In this way, reconstructed data can be obtained.
In addition, the holographic recording and reconstructing apparatus 50 includes an optical system used for controlling the position of a recording and reconstructing point (a laser spot position) on the rotating holographic recording medium HM. More specifically, as shown in FIG. 8, the optical system that includes a sub-laser 21, a lens 22, a collimator lens 23, a beam splitter 24, a condenser lens 25, a lens 26, and a photodetector 27 is employed.
In order to control the position of the recording and reconstructing point using a tracking servo, recording and reconstructing apparatuses that support existing optical discs, such as CDs and DVDs, use a laser beam for recording and reconstructing data. The reason why such a common laser beam can be used for recording and reconstructing data and controlling the position of the recording and reconstructing point is because a recording layer of the optical discs has a clearly defined threshold value for recording power.
However, the characteristics of holographic recording media are different from those of existing optical discs. That is, currently, photopolymers are promising recording materials for the holographic recording medium HM, although photopolymers have no clearly defined threshold value for recording power. Accordingly even when, as in the case of existing optical discs, low-power laser light beam is emitted to a photopolymer disc, monomers may be changed to polymers in some portions. Thus, the recording performance of these portions is highly likely to be degraded.
Accordingly, in holographic recording and reconstructing methods, in order to perform position control, such as tracking servo control used for existing optical discs, a laser light beam having a wavelength different from that of a laser light beam used for recording and reconstructing data is used. Thus, reaction producing the polymer is reliably prevented.
As shown in FIG. 8, the sub-laser 21 is designed so as to emit a laser beam having a wavelength different from that of a laser light beam used for recording and reconstructing data. For example, the sub-laser 21 emits a red laser beam having a wavelength of about 650 nm, which is used for DVDs.
A light beam emitted from the sub-laser 21 is made incident on the dichroic mirror 14 via the lens 22, the collimator lens 23, and the beam splitter 24.
The dichroic mirror 14 has wavelength selectivity so as to transmit a recording and reconstructing laser beam emitted from the main laser 2 (the wavelength is about 405 nm) and reflect a position control laser beam emitted from the sub-laser 21 (the wavelength is about 650 nm). Accordingly, the position control laser beam made incident in the above-described manner is reflected by the dichroic mirror 14. Thereafter, the position control laser beam travels along the light path described above for the recording and reconstructing laser beam and passes through the objective lens 16. The position control laser beam is then emitted onto the holographic recording medium HM.
In the holographic recording medium HM, a first reflecting film is provided under a recording layer in which a hologram is recorded. The first reflecting film has wavelength selectivity so as to reflect the recording and reconstructing laser beam and pass the position control laser beam therethrough. A groove for position control is formed in an underlayer of the first reflecting film. A second reflecting film is provided under the groove layer.
Accordingly, the position control laser beam is emitted to the holographic recording medium HM, as described above. The position control laser beam passes through the first reflecting layer and reaches the groove layer located under the first reflecting layer. The position control laser beam that has reached the groove layer is then reflected by the underlying second reflecting film. The reflected light beam becomes a returning light beam and travels along the above-described path. The returning light beam is then led to the dichroic mirror 14.
Thereafter, the returning light beam for position control output from the holographic recording medium HM is reflected by the dichroic mirror 14 and is led to the beam splitter 24. The returning light beam is reflected by the beam splitter 24. Subsequently, the returning light beam travels through the condenser lens 25 and the lens 26. Finally, the returning light beam is made incident on the photodetector 27.
In this way, the returning light beam can be detected by the photodetector 27. Thereafter, a servo circuit (not shown) can perform position control, such as various servo control and access control of data at a specified address on the basis of a detection signal output from the photodetector 27.
In addition, in the hologram recording and reconstructing method, when, as shown in FIG. 8, data is recorded and reconstructed on and from the rotating holographic recording medium HM, a laser beam is scanned at predetermined intervals in such a way that the laser beam is emitted at the same location for a predetermined period of time. That is, by scanning the laser beam in this way, interference fringes are more reliably formed when data is recorded. In addition, when data is reconstructed, data can be more reliably read out by increasing the intensity of a detection light beam. Such a function of scanning a laser beam at predetermined intervals and emitting the laser beam onto the same location on the medium for a predetermined period of time is called an “image stabilization function”.
As shown in FIG. 8, the existing holographic recording and reconstructing apparatus 50 includes the image stabilization function unit 53 for realizing such an image stabilization function.
The image stabilization function unit 53 includes an AOM 51, the AOD 4, and a control unit 52 that controls the AOM 51 and the AOD 4.
For example, the AOM 51 is driven using a high-frequency signal of a hundred and several tens of megahertz. The AOM 51 includes a device (an acousto-optic device) whose transmittance changes in accordance with a change in the amplitude of the high-frequency signal. That is, the change in transmittance realizes a shutter function.
Like the AOM 51, the AOD 4 is driven using a high-frequency signal. The AOD 4 includes an acousto-optic device that changes the deflection angle of a light beam in accordance with a change in the frequency of the high-frequency signal. The AOD 4 functions as a module that scans a laser light beam by controlling the deflection angle.
In the image stabilization function, in order to sequentially emit a laser beam onto predetermined locations for a predetermined period of time, a blanking period is necessary in which a laser spot is moved from one location to another location. If the laser beam is continuously emitted during this blanking period, the recording material reacts to no small extent. In particular, when data is recorded, a residual image caused by the movement of a laser spot may be added to a recorded hologram (a diffraction grating), thereby generating noise.
Therefore, in order to provide an image stabilization function, a unit (a shutter) for preventing reaction of a recording material by significantly decreasing the transmittance of a laser beam during the blanking period can be provided in addition to a unit for scanning a laser beam. More specifically, the existing image stabilization function unit 53 includes the AOM 51 serving as such a shutter.
As shown in FIG. 8, in order to realize the image stabilization function, the control unit 52 controls the AOM 51 and the AOD 4 so that the deflection angle and the transmittance of the laser beam are changed. More specifically, a saw-tooth wave driving signal is provided to the AOD 4 so that a scanning operation is performed at predetermined intervals. In addition, a square wave driving signal is provided to the AOM 51 so that a laser beam passes through the AOM 51 during the scanning period of the AOD 4 and the laser beam is blocked during the blanking period, which is a period between the scanning periods.
One of the related art documents is Japanese Unexamined Patent Application Publication No. 2002-341732.